Electro-chemical sensor

ABSTRACT

An electro-chemical sensor is described having one or more redox species sensitive to a certain analyte and one or more redox species that are insensitive to that analyte, for the purpose of making measurements in a downhole environment in aquifers or oilfield reservoirs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of, and claims the benefitsof priority from application Ser. No. 10/585,263, entitled“ELECTRO-CHEMICAL SENSOR,” filed in the United States of America.Application Ser. No. 10/585,263 is a national phase application (371) ofApplication Number PCT/GB2004/005397, entitled “ELECTRO-CHEMICALSENSOR,” filed under the PCT on Dec. 22, 2004 and claims priority toGB0400325.7, entitled “ELECTRO-CHEMICAL SENSOR,” filed in the UnitedKingdom on Jan. 8, 2004. Accordingly, this application also claims thebenefits of priority from Application Number GB0400325.7.

All of which are commonly assigned to assignee of the present inventionand hereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to a chemical sensor forsensing properties of a fluids. More specifically, but not by way oflimitation, certain embodiments of the present invention may provide anelectro-chemical sensor for pH and ion content analysis of fluids.Merely by way of example, some aspects of the present invention may beused in a downhole environment, such as aquifer and oilfield reservoirand may be used to analyse substances produced from and/or found in anaquifer or an oilfield reservoir.

Analyzing samples representative of downhole fluids is an importantaspect of determining the quality and economic value of a hydrocarbonformation. Similarly, analyzing properties of liquids associated with anaquifer may be important in aquifer analysis in the hydrocarbon, waterproduction industries and/or resource management.

In the hydrocarbon industry, analysis operations may obtain an analysisof downhole fluids usually through wireline logging using a formationtester such as the MDT™ tool of Schlumberger Oilfield Services. However,more recently, it was suggested to analyze downhole fluids eitherthrough sensors permanently or quasi-permanently installed in a wellboreor through sensors mounted on the drillstring. The latter method, ifsuccessfully implemented, has the advantage of obtaining data whiledrilling, whereas the former installation could be part of a controlsystem for wellbores and hydrocarbon production therefrom.

To obtain an estimate of the composition of downhole fluids, the MDTtools may use an optical probe to estimate the amount of hydrocarbons inthe samples collected from the formation. Other sensors use resistivitymeasurements to discern various components of the formations fluids.

Particularly, knowledge of downhole formation (produced) water chemistryis needed to save costs and increase production at all stages of oil andgas exploration and production. Knowledge of particularly the waterchemistry is important for a number of key processes of the hydrocarbonproduction, including:

-   -   Prediction and assessment of mineral scale and corrosion;    -   Strategy for oil/water separation and water re-injection;    -   Understanding of reservoir compartmentalization/flow units;    -   Characterization of water break-through;    -   Derivation of the water cut R_(w); and    -   Evaluation of downhole the H₂S partition the oil and or water        (if used for H₂S measurements).

Some chemical species dissolved in water (like, for example, Cl⁻ andNa⁺) do not change their concentration when removed to the surfaceeither as a part of a flow through a well, or as a sample takendownhole. Consequently information about their quantities may beobtained from downhole samples and in some cases surface samples of aflow. However, the state of chemical species, such as H⁺(pH=−log[concentration of H⁺]), CO₂, or H₂S may change significantlywhile tripping to the surface. The change occurs mainly due to adifference in temperature and pressure between downhole and surfaceenvironment. In case of sampling, this change may also happen due todegassing of a sample (seal failure), mineral precipitation in asampling bottle, and (especially in case of H₂S)—a chemical reactionwith the sampling chamber. It should be stressed that pH, H₂S, or CO₂are among the most critical parameters for corrosion and scaleassessment. Consequently it is of considerable importance to know theirdownhole values precisely.

The concentration of protons or its logarithm pH can be regarded as themost critical parameter in water chemistry. It determines the rate ofmany important chemical reactions as well as the solubility of chemicalcompounds in water, and (by extension) in hydrocarbon.

Hence, there is and will continue to be a demand for downhole chemicalmeasurements. However, no downhole chemical measurements actuallyperformed in an oil and gas producing well have been reported so far,though many different methods and tools have been proposed in therelevant literature.

General downhole measurement tools for oilfield applications are knownas such. Examples of such tools are found in the U.S. Pat. Nos.6,023,340; 5,517,024; and 5,351,532 or in the International PatentApplication WO 99/00575. An example of a probe for potentiometricmeasurements of ground water reservoirs is further published as:Solodov, I. N., Velichkin, V. I., Zotov, A. V. et al. “Distribution andGeochemistry of Contaminated Subsurface Waters in Fissured VolcanogenicBed Rocks of the Lake Karachai Area, Chelyabinsk, Southern Urals” in:Lawrence Berkeley Laboratory Report 36780/UC-603(1994b), RAC-6, Ca, USA.

The known state of the art in the field of high temperaturepotentiometric measurements and tool is described for example in thepublished UK patent application GB-2362469 A.

A number of chemical analysis tools are known from chemical laboratorypractice. Such known analysis tools include for example the varioustypes of chromatography, electro-chemical and spectral analysis.Particularly, the potentiometric method has been widely used for themeasurements of water composition (pH, Eh, H₂S, CO₂, Na³⁰ , Cl⁻ etc.)both in the laboratory and in the field of ground water quality control.U.S. Pat. No. 5,223,117 discloses a two-terminal voltammetricmicrosensor having an internal reference using molecular self-assemblingto form a system in which the reference electrode and the indicatorelectrode are both on the sensor electrode. The reference molecule isdescribed as a redox system that is pH-insensitive, while the indicatormolecule is formed by a hydro-quinone based redox system having apotential that shifts with the pH. Both, reference molecule andindicator molecule layers are prepared by self-assembly on gold (Au)microelectrodes. In the known microsensor, a pH reading is derived frompeak readings of the voltammograms.

The laboratory systems, however, are often not suitable for wellboreapplication with demands for ruggedness, stability and low maintenanceand energy consumption being rarely met.

It is therefore an object of the present invention to provide apparatusand methods to perform electro-chemical measurements in hydrocarbonwells during drilling and production. More specifically, it is an objectof the present invention to provide robust sensors for ion selectiveelectro-chemical measurements, in particular pH measurements.

SUMMARY OF THE INVENTION

Some embodiments of the present invention may provide anelectro-chemical sensor comprising one or more redox species that aresensitive to an analyte and one or more redox centers that areinsensitive to the analyte. In certain aspects, the sensor is configuredfor use in a downhole environment, including but not limited to,hydrocarbon reservoirs and/or aquifers.

In one embodiment of the present invention, the sensor may comprise aredox system, based for example on anthraquinone redox chemistry.

The substrate onto which the redox system is mounted may be based oncarbon in one of its elementary forms such as graphite, carbon powder,diamond. In a variant of the invention, the substrate may be derivatisednanotubes, including multi-walled nanotubes.

An electro-chemical technique using a method or sensor in accordancewith the present invention may be applied for example as part of aproduction logging tool, an open hole formation tester tool (such as theModular Dynamic Tester, MDT™), an aquifer analyzing tool and/or thelike. In certain aspects, the technique according to certain embodimentsof the present invention may provide a downhole real-time water samplevalidation or downhole pH measurement which may be used for predictingmineral scale, corrosion assessment and/or the like.

These and other features of the invention, embodiments and variantsthereof, possible applications and advantages may become appreciated andunderstood by those skilled in the art from the following detaileddescription and drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic diagram of the main elements of a knownvoltametric sensor;

FIG. 2 A-C shows a schematic diagram of the main elements of a knownelectro-chemical microsensor and its operation;

FIG. 3 shows a schematic diagram of a known downhole probe usingpotentiometric sensors;

FIG. 4A illustrates the surface structure of a measuring electrode inaccordance with an example of the invention;

FIG. 4B illustrates the surface structure of a measuring electrode withinternal reference electrode in accordance with another example of theinvention;

FIG. 4C illustrates the redox reaction of a measuring electrode inaccordance with another example of the invention using multi-walledcarbon nanotubes;

FIG. 4D illustrates the redox reaction of a measuring electrode withinternal reference electrode in accordance with another example of theinvention using multi-walled carbon nanotube;

FIG. 4E illustrates the geometrical surface layout of the electrode ofFIG. 4B;

FIG. 5 is a perspective view, partially cut-away, of a sensor inaccordance with an example of the present invention in a downhole tool;

FIG. 6 shows voltammograms recorded from an electro-chemical microsensorin accordance with the present invention at three different pH values;

FIG. 7A illustrates the shift of the peak potential for anthraquinone,diphenyl-p-phenylenediamine and a combination of the two redox systems;

FIGS. 7B-C are plots of peak potential against pH for the redox systemsof FIGS. 4C and 4D, respectively, over the pH range pH 1.0 to pH 12.0 at293 K at various conditions;

FIG. 7D illustrates change in peak potential with temperature.

FIG. 7E illustrates the effect of varying pH at room temperature formolecular anthraquinone in the solution phase versus the AQ-MWCNTsimmobilized onto a bppg electrode.

FIG. 8 illustrates an example of a sensor in accordance with theinvention as part of a wireline formation testing apparatus in awellbore;

FIG. 9 shows a wellbore and the lower part of a drill string includingthe bottom-hole-assembly, with a sensor in accordance with theinvention; and

FIG. 10 shows a sensor located downstream of a venturi-type flowmeter,in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The theory of voltammetry and its application to surface watermeasurements at ambient temperatures are both well developed. The methodis based on the measurement of the electromotive force (e.m.f.) orpotential E in a potentiometric cell which includes measuring andreference electrodes (half-cells).

FIG. 1 shows the general components of a known voltammetric cell 10. Ameasuring electrode 11 is inserted into a solution 13. This electrodeconsists of an internal half element (for example, Ag wire covered by anAgCl salt) in a solution of a fixed pH (for example, 0.1M HCl in some pHelectrodes), and an ion-selective membrane 111 (like glass H⁺ selectivemembrane in pH glass electrode). The reference electrode 12 alsocontains an internal half-element (typically the same AgCl; Ag) insertedin a concentrated KCl (for example 3M) solution/gel saturated with Ag⁺,which diffuses (or flows) through the reference (liquid) junction 121.

The ion-selective electrode 11 measures the potential that arisesbecause of the difference in activity or concentration of acorresponding ion (H⁺ in case of pH) in the internal solution and in themeasured solution. This potential is measured against the referencepotential on the reference electrode 12, which is fixed because of aconstant composition of a reference solution/gel. The electrodes may beseparated (separate half cells), or combined into one (“combinationelectrode”).

The measured e.m.f. is an overall function of the temperature and theactivity of an ith ion, to which the measuring electrode is selective:E=E ⁰+(k*T)*log(a _(i)),  [1]where E is the measured electromotive force (e.m.f.) of the cell (allpotentials are in V), a_(i) corresponds to the activity of the ith ionand is proportional to its concentration. E⁰ is the standard potential(at temperature T) corresponding to the E value in a solution with theactivity of ith ion equal to one. The term in parenthesis is theso-called Nernstian slope in a plot of E as a function of log(a_(i)).This slope (or the constant “k”) together with the cell (electrode)constant (E⁰) is experimentally determined via a calibration procedureusing standard solutions with known activities of ith ion. For goodquality undamaged electrodes this slope should be very close to thetheoretical one, equal to (R*T/F*z), where F is the Faraday constant(96485 kJ/mole), R is the gas constant (8.313 j/mole K), z_(i) is thecharge of ith ion.

The Nernst equation [1] can be rewritten for pH sensors, i.e. log a(H⁺)asE _(0.5) =K−(2.303RTm/nF)pH  [2]where E_(0.5) is the half-wave potential of the redox system involved, Kis an arbitrary constant, R is the ideal gas constant, m is the numberof protons and n is the number of electrons transferred in the redoxreaction.

The microsensor of U.S. Pat. No. 5,223,117 is illustrated in FIG. 2.FIG. 2A. shows a schematic electro-chemical sensor with a counterelectrode 21 and a relatively much smaller (by a factor of 1000) Ausubstrate 22 that carries two molecular species M and R. The R speciesforms an inert reference electrode, and species M is an indicatorelectrode with specific receptors or sensitivity for a third species L.The schematic linear sweep voltammogram in the upper half of FIG. 2Cshows the difference in the current peaks for the oxidization in thenormal state. When the third species L binds to M (FIG. 2B), thisdifference increases as illustrated by the shift of peaks in the lowerhalf of FIG. 2C, thus providing a measure for the concentration of L inthe solution surrounding the sensor. In the context of the presentinvention, it is important to note that the R is specifically selectedto be insensitive to the species L, e.g. pH.

In FIG. 3, there are schematically illustrated elements of a knowndownhole analyzing tool 30 as used by Solodov et al (see background).The body of the tool 30 is connected to the surface via a cable 31 thattransmits power and signals. A computer console 32 controls the tool,monitors its activity and records measurements. The tool 30 includes asensor head with at number of selective electro-chemical probes 33 eachsensitive to a different molecular species. Also housed in the body ofthe tool are further actuation parts 34 that operate the head, a testsystem 35 and transceivers 36 to convert measurements into a data streamand to communicate such data stream to the surface. The electrodes arelocated at the bottom part of the probe and include those for pH, Eh (orORP), Ca²⁺ (pCa), Na⁺ (pNa), S²⁻ (pS), NH₄ ⁺ (pNH₄), and referenceelectrode (RE). H₂S partial pressure may be calculated from pH and pSreadings.

In the following aspects and elements of the present invention aredescribed in detail.

The present invention introduces a new molecular system in which theredox features of two molecules are combined, thus leading to aconsiderably higher accuracy and, in turn, downhole deployability.

In a preferred embodiment for a pH sensitive sensor an anthraquinone ishomogenously derivatised onto carbon particles (AQC)

The AQC system is derived using 2 g of carbon powder (1.5 μm in meandiameter) mixed with a 10 cm³ solution containing 5 mM Fast Red AL Salt(Anthraquinone-1-diazonium chloride) to which 50 mM hypophosphorous acid(50%) is added. The reaction is allowed to stand with occasionalstirring at 5° C. for 30 minutes, after which it is filtered by watersuction. Excess acid is removed by washing with distilled water and withthe powder being finally washed with acetonitrile to remove anyunreacted diazonium salt in the mixture. It is then air dried by placinginside a fume hood for a period of 12 hours and finally stored in anairtight container.

In a similar manner, phenanthrenequinone (PAQ)

Is prepared as a second molecular species to undergo a redox reaction

Alternatively, N,N′-diphenyl-p-phenylenediamine (DPPD) spiked ontocarbon particles undergoes a redox process as shown below:

The bonding of DPPD onto carbon is achieved by mixing 4 g of carbonpowder with 25 mL of 0.1M HCl+0.1M KCl and 20 mM DPPD solution inacetone. The reaction mixture is stirred continuously for 2 hours in abeaker and then filtered after which it was washed with distilled waterto remove excess acid and chloride. It is then air dried by placinginside a fume hood for 12 hours and finally stored in an airtightcontainer.

In a static environment where the sensor surface is not exposed to aflow, it is possible to immobilize water insoluble DPPD crystalsdirectly onto the electrode surface. However in the wellbore environmentit is preferred to link the sensitive molecules via a chemical bond tosuch surface.

The derivatised carbon powders are immobilized onto a basal planepyrolytic graphite (BPPG) electrode prior to voltammetriccharacterization following a procedure described by Scholz, F. andMeyer, B., “Voltammetry of Solid Microparticles Immobilised on ElectrodeSurfaces in Electroanalytical Chemistry” ed. A. J. Bard, and I.Rubenstein, Marcel Dekker, New York, 1998, 20, 1. Initially theelectrode is polished with glass polishing paper (H00/240) and then withsilicon carbide paper (P1000C) for smoothness. The derivatised carbonsare first mixed and then immobilized onto the BPPG by gently rubbing theelectrode surface on a fine qualitative filter paper containing thefunctionalized carbon particles.

The resulting modified electrode surface is schematically illustrated byFIG. 4A showing an electrode 41 with bonded DPPD and AQC.

It is further advantageous to add an internal pH reference involving apH independent redox couple to increase the stability of anyvoltammetric reading, hence circumventing uncertainties caused by driftof the external reference electrode. In the configuration, the sensorincludes two reference electrodes.

A suitable reference molecule is, for example, K₅Mo(CN)₈ or variousferrocene containing molecules which both have a stable redox potential(K₅Mo(CN)₈ at around 521 mV) that is sufficiently separated fromexpected shifting of redox signals of the two indicator species over thepH range of interest. As shown in Table 1 that both the oxidation andreduction potentials of K₅Mo(CN)₈ are fairly constant across the entirepH range of interest.

TABLE 1 pH AQ_(OX) AQ_(RED) DPPD_(OX) DPPD_(RED) Mo—_(OX) Mo—_(RED) 4.6−0.440 −0.448 0.202 0.224 0.524 0.524 6.8 −0.576 −0.580 0.094 0.0820.528 0.522 9.2 −0.710 −0.674 −0.204 −0.372 0.512 0.508

The Mo-based reference species can be retained in the solid substratevia ionic interactions with co-existing cationic polymer, such as poly(vinyl pyridine), that was spiked into the solid phase. Other pHindependent species, such as ferrocyanide are less suitable as the redoxpeaks are obscured by the signals of the measuring redox system.

In FIG. 4B the electrode 42 carries bonded molecules AQC and PAQtogether with PVF as an internal reference molecule.

The most common forms of conducting carbon used in electrode manufactureare glassy carbon, carbon fibres, carbon black, various forms ofgraphite, carbon paste and carbon epoxy. One further form of carbon,which has seen a large expansion in its use in the field ofelectrochemistry since its discovery in 1991 is the carbon nanotube(CNT). The structure of CNTs approximates to rolled-up sheets ofgraphite and can be formed as either single or multi-walled tubes.Single-walled carbon nanotubes (SWCNTs) constitute a single, hollowgraphite tube. Multi-walled carbon nanotubes (MWCNTs) on the other handconsist of several concentric tubes fitted one inside the other.

The above activation methods for binding a redox active species tographite or carbon surfaces can be extended via the chemical reductionof aryldiazonium salts with hypophosphorous acid, to include thecovalent derivatization of MWCNTs by anthraquinone-1-diazonium chlorideand 4-nitrobenzenediazonium tetrafluoroborate. This results in thesynthesis of 1-anthraquinonyl-MWCNTs (AQ-MWCNTs) and4-nitrophenyl-MWCNTs (NP-MWCNTs) as shown in FIGS. 4C and 4D,respectively. The respective substrates 46 and 47 are multi-walledcarbon nanotubes.

The preparation process of the derivatised MWCNT involves the followingsteps: first 50 mg of MWCNTs are stirred into 10 cm³ of a 5 mM solutionof either Fast Red AL (anthraquinone-1-diazonium chloride) or Fast RedGG (4-nitrobenzenediazonium tetrafluoroborate), to which 50 cm³ ofhypophosphorous acid (H₃PO₂, 50% w/w in water) is added. Next thesolution is allowed to stand at 5° C. for 30 minutes with gentlestirring. After which, the solution is filtered by water suction inorder to remove any unreacted species from the MWCNT surface. Furtherwashing with deionized water is carried out to remove any excess acidand finally with acetonitrile to remove any unreacted diazonium saltfrom the mixture. The derivatised MWCNTs are then air-dried by placingthem inside a fume hood for a period of 12 hours after which they arestored in an airtight container prior to use. Untreated multi-wallednanotubes can be purchased from commercial vendors, for example fromNano-Lab Inc of Brighton, Mass., USA in 95% purity with a diameter of30+/−15 nm and a length of 5-20 μm.

The reduction of diazonium salts using hypophosphorous acid asdemonstrated is a versatile technique for the derivatization of bulkgraphite powder and MWCNTs. This has the advantage over previous methodsinvolving the direct electro-chemical reduction of aryldiazonium saltsonto the electrode surface, as the chemically activated method allowsthe possibility for inexpensive mass production of chemicallyderivatised nanotubes for a variety of applications. Furthermore thederivatization of MWCNTs proffers the possibility of sensorminiaturization down to the nano-scale.

Another way of immobilizing the redox active compounds onto the workingelectrode terminal would be by packing a mixture of the compounds andcarbon powder effectively into a recessed working electrode cavitywithout a binding substance. The carbon powder could be mixed with thepH-sensitive and reference chemicals and ground finely with a mortar andpestle. Then the empty recess might be filled with the powder mix whichwould be mechanically compacted. The resulting void in the workingelectrode recess would then be refilled and compacted again. This wouldbe repeated several times until the recess is full. The material wouldbe pressed such that the particles are packed into a dense matrix. Inorder to achieve stability against liquid or gas flow across thesurface, the carbon press electrode might be protected by a porous frit,a fiber mesh, a non-conducting wire mesh or a porous polymer film.

Although packing of the redox active compounds into a single electrodearea (as discussed above) provides a means of forming the sensor it canbe envisaged that immobilization of two or more species into variousdistinct electrodes may provide improved signals and more facilemanufacturing. This can be especially thought of when the compounds arechemically attached to the electrode surface via a covalent linkage. Inthis case a single monolayer of compounds will be formed on the surface.It can be envisaged that if the pH sensitive and pH insensitivecompounds are bulky or undergo differing immobilization rates thenformation of the monolayer will favor one or other of the compounds suchthat the signal is dominated by a single compound and hence the sensoris inoperable. In these cases immobilization of each compound ontoseparate electrodes would overcome the problem, as the immobilizationprocedure for each would not be under competitive control. It cantherefore be proposed that a sensor in which two or more workingelectrodes, with different electroactive species immobilized on eachsurface, is utilized and cross connected such that only a singlevoltammetric sweep is required.

In FIG. 4E there is shown a possible geometric configuration or layoutfor the sensor surface 40 which is exposed to the wellbore fluid. Thesurface includes a working electrode 43 as described in FIG. 4A or 4B,together with the (external) reference electrode 44 and a counterelectrode 45.

A schematic of a microsensor 50 incorporating a modified surfaceprepared in accordance with the procedure described above is shown inFIG. 5. The body 51 of the sensor is fixed into the end section of anopening 52. The body carries the electrode surface 511 and contacts 512that provide connection points to voltage supply and measurement througha small channel 521 at the bottom of the opening 52. A sealing ring 513protects the contact points and electronics from the wellbore fluid thatpasses under operation conditions through the sample channel 53.

It is an advantage of the new sensor to include two measuring orindicator electrodes or molecules measuring two e.m.f or potentials withreference to the same reference electrode and being sensitive to thesame species or molecule in the environment. As a result the sensitivitytowards a shift in the concentration of the species increases. Using theabove example of AQC and DPPD and the pH (or H⁺ concentration, theNernst equation applicable to the new sensor is the sum of the equationsdescribing the individual measuring electrodes. Thus, combining the halfwave potential E_(0.5) (AQC) for anthraquinoneE _(0.5)(AQC)=K(AQC)−(2.303RTm/nF)pH  [3]with the half wave potential E_(0.5)(DPPD) forN,N′-diphenyl-p-phenylenediamineE _(0.5)(DPPD)=K(DPPD)−(2.303RTm/nF)pH  [4]yields the half wave potential E_(0.5)(S) for the combined system:E _(0.5)(S)=E _(0.5)(AQC)+E_(0.5)(DPPD)=(K(AQC)+K(DPPD))−2*(2.303RTm/nF)pH=K(S)−2*(2.303RTm/nF)pH  [5]

Where K(S) is the sum of the two constants K(AQC) and K(DPPD). As theshift of the potential with a change in pH depends on the second term,the (theoretical) sensitivity of the sensor has doubled.

The use of a further (third) redox system sensitive to the same specieswould in principle increase the sensitivity further. As the methoddetects shifts in the peak location of the voltammogram, however, moreefforts are anticipated to be required to resolve overlapping peaks insuch a three-molecule system.

FIG. 6 shows results in a range of pH solutions (pH 4.6, 0.1M aceticacid+0.1M sodium acetate buffer; pH 6.8, 0.025M disodium hydrogenphosphate+0.025M potassium dihydrogen phosphate buffer; pH 9.2, 0.05Mdisodium tetraborate buffer). The figure presents the correspondingsquare wave voltammograms when the starting potential was sufficientlynegative to have both DPPD and AQ in their reduced forms.

FIG. 7A depicts the relationship between the redox potential and pH forboth the DPPD (▪) and AQ (♦). The plot reveals a linear response from pH4 to 9 with a corresponding gradient of ca 59 mV/pH unit (at 25° C.)which is consistent with an n electron, m proton transfer where n and mare likely to be equal to two. By combining the two individual curves ina manner as described in equation [5], a new function (▴) is derivedwith a superior sensitivity for the species to be detected.

For the two activated MWCNT species described above, the peak potentialusing cyclic voltammetry (CV) is found to be pH-dependant. Thisvoltammetric behavior is consistent with previous studies of carbonpowder covalently modified with 1-anthraquinonyl groups and can beattributed to the two-electron, two-proton reduction/oxidation of the1-anthraquinonyl moiety to the corresponding hydroquinone species.

When NB-MWCNTs is studied a more complicated voltammetric pattern can beobserved. Upon first scanning in a reductive fashion a large,electro-chemically irreversible peak is observed (labeled as system I),the exact peak potential of which depends on the pH studied. When thescan direction is reversed and swept in an oxidative fashion a new peakat more positive potentials than the irreversible peak is observed,which upon repeat cycling was found to behave in an electro-chemicallyreversible fashion as the corresponding reduction wave was observed.This system is labeled as system II. Again the exact peak potential ofsystem II is found to vary with the pH studied. This behavior isconsistent with the reduction mechanism, of the nitro moiety in aqueousmedia as exemplified by nitrobenzene in FIG. 4D. It is worth noting thatall subsequent characterization procedures for NB-MWCNTs are carried outon system II, which corresponds to the reversiblearylnitroso/arylhydroxylamine couple, after several initial scans areperformed to form this redox couple.

When investigating the effect of pH of AQ-MWCNTs and NB-MWCNTs over therange pH 1.0 to pH 12.0 using CV and square wave voltammetry (SWV) atroom temperature as well as the behavior of AQ-MWCNTs at elevatedtemperatures up to 70° C. SWV was used because it provides us with asharp, well-defined peak in a single sweep. As concomitant protonloss/gain occurs on oxidation/reduction of AQ-MWCNTs or NB-MWCNTs (seeFIGS. 4C and 4D respectively) the peak potential depends on the localproton concentration, i.e. pH, as described by the Nernst equation [6]:

$\begin{matrix}{E_{peak} = {E_{formal}^{0} - {\frac{2.3\mspace{14mu}{RTm}}{nF}{pH}}}} & \lbrack 6\rbrack\end{matrix}$where m and n, the number of protons and electrons transferredrespectively, are both likely to be equal to two in the case ofAQ-MWCNTs and the arylnitroso/arylhydroxylamine couple in the case ofNB-MWCNTs. The formulation [6] of the Nernst equation is equivalent tothose of equations [1] and [2].

At room temperature the peak potentials for both AQ-MWCNTs and NB-MWCNTsare found to shift to more negative potentials with increasing pH aspredicted. A corresponding plot of peak potential against pH was foundto be linear over the entire pH range studied in all cases (see FIGS. 7Band 7C, respectively) and a comparison of the gradient of the plots ofpeak potential vs. pH are found to be close to the ideal value of 58.1mV/pH unit with the exception of the irreversible peak (system I) forNB-MWCNTs which was found to shift by only 37.6 mV/pH unit.

The response of AQ-MWCNTs to pH at elevated temperatures up to 70° C. isstudied using SWV. Note that the pH of the solutions used may vary withtemperature, and so to this end three IUPAC buffers with a known pH ateach temperature studied were employed. These are the pH 4.6, pH 6.8 andpH 9.2 buffers. The Nernst equation predicts that the peak potentialshould shift to more negative values as the temperature is increased dueto the temperature dependence of the formal potential (E_(peak) ⁰). FIG.7D does indeed reveal that as the temperature is increased the peakpotential is shifted to more negative values. However, in contrast tothe behavior of carbon powder covalently derivatised with theanthraquinonyl moiety (AQcarbon) where the peak currents increasesteadily with increasing temperature after an initial increase in peakcurrent up to ca 40° C., the peak currents for AQ-MWCNTs graduallydecreases with increasing temperature. This behavior has also beenpreviously observed for MWCNT agglomerates at elevated temperatures. Thetemperature invariance of derivatised MWCNTs is not fully understood buthas a potential advantage for pH sensors which are required for use inelevated temperature environments.

In FIG. 7E there is illustrated the effect of varying pH at roomtemperature for molecular anthraquinone in the solution phase versus theAQ-MWCNTs immobilized onto a bppg electrode. 1 mM anthraquinonesolutions are prepared at each pH and studied using cyclic voltammetryat a bare bppg electrode. The variation of peak potential with pH forboth cases over the pH range 1.0 to 14.0 are studied with additionalexperiments carried out at pH 10.5, pH 13.0 and pH 14.0. The plot ofpeak potential versus pH for both 1 mM anthraquinone in solution and forthe immobilized AQ-MWCNTs reveals that, in the case of AQ-MWCNTs, alinear response is observed over the entire pH range studied. Howeverfor the anthraquinone in the solution phase, the plot is no longerlinear above ca. pH 10.5 (FIG. 7E). This can be attributed to the pKafor the removal of the first proton, pKa₁, of the reduced form ofanthraquinone (see FIG. 4C) in solution being ca. pKa₁=10. The pKa forthe removal of the second proton is ca pKa₂=12. At higher pHs than pH 10the reduced form of anthraquinone may be deprotonated causing a changein the variation of peak potential with pH. No such deviation fromlinearity is observed for the AQ-MWCNTs. From this it can be concludedthat derivatization onto the surface of the MWCNTs may change the pK_(a)of the anthraquinonyl moiety. This clearly demonstrates thatderivatization onto MWCNTs proves advantageous to the analytical sensingof pH as the pH window for use is favorably widened for derivatisedAQ-MWCNTs compared to free anthraquinone in solution.

Analysis of the peak potential as a function of pH at each temperatureshows good agreement between the experimental and theoreticallypredicted values thereby showing the mechanism can be readily used as asimple, inexpensive pH probe, which works over a wide range oftemperatures.

The novel probe may be placed inside various wellbore tools andinstallations as described in the following examples.

In FIGS. 8-11 the sensor is shown in various possible downholeapplications.

In FIG. 8, there is shown a formation testing apparatus 810 held on awireline 812 within a wellbore 814. The apparatus 810 is a well-knownmodular dynamic tester (MDT, Mark of Schlumberger) as described in theco-owned U.S. Pat. No. 3,859,851 to Urbanosky U.S. Pat. No. 3,780,575 toUrbanosky and U.S.Pat. No. 4,994,671 to Safinya et al., with this knowntester being modified by introduction of an electro-chemical analyzingsensor 816 as described in detail above (FIG. 8). The modular dynamicstester comprises body 820 approximately 30m long and containing a mainflowline bus or a flowline 822. The sensor 816 communicates with theflowline 822 via opening 817. In addition to the sensor 816, the testingapparatus comprises an optical fluid analyser 830 within the lower partof the flowline 822. The flow through the flowline 822 is driven bymeans of a pump 832 located towards the upper end of the flowline 822.Hydraulic arms 834 and counterarms 835 are attached external to the body820 and carry a sample probe tip 836 for sampling fluid. The base of theprobing tip 836 is isolated from the wellbore 814 by an o-ring 840, orother sealing devices, e.g. packers.

Before completion of a well, the modular dynamics tester is lowered intothe well on the wireline 812. After reaching a target depth, i.e., thelayer 842 of the formation which is to be sampled, the hydraulic arms834 are extended to engage the sample probe tip 836 with the formation.The o-ring 840 at the base of the sample probe 836 forms a seal betweenthe side of the wellbore 844 and the formation 842 into which the probe836 is inserted and prevents the sample probe 136 from acquiring fluiddirectly from the borehole 814.

Once the sample probe 836 is inserted into the formation 842, anelectrical signal is passed down the wireline 812 from the surface so asto start the pump 832 and the sensor systems 816 and 830 to beginsampling of a sample of fluid from the formation 842. Theelectro-chemical detector 816 is adapted to measure the pH andion-content of the formation effluent.

A bottle (not shown) within the MDT tool may be filled initially with acalibration solution to ensure in-situ (downhole) calibration ofsensors. The MDT module may also contain a tank with a greater volume ofcalibration solution and/or of cleaning solution which may periodicallybe pumped through the sensor volume for cleaning and re-calibrationpurposes.

Electro-chemical probes in an MDT-type downhole tool may be used for theabsolute measurements of downhole parameters which significantly differfrom those measured in samples on the surface (such as pH, Eh, dissolvedH₂S, CO₂). This correction of surface values are important for waterchemistry model validation.

A further possible application of the novel sensor and separation systemis in the field of measurement-while-drilling (MWD). The principle ofMWD measurements is known and disclosed in a vast amount of literature,including for example U.S. Pat. No. 5,445,228, entitled “Method andapparatus for formation sampling during the drilling of a hydrocarbonwell”.

In FIG. 9, there is shown a wellbore 911 and the lower part of a drillstring 912 including the bottom-hole-assembly (BHA) 910. The BHA carriesat its apex the drill bit 913. It includes further drill collars thatare used to mount additional equipment such as a telemetry sub 914 and asensor sub 915. The telemetry sub provides a telemetry link to thesurface, for example via mud-pulse telemetry. The sensor sub includesthe novel electro-chemical analyzing unit 916 as described above. Theanalyzing unit 916 collects fluids from the wellbore via a small recess917 protected from debris and other particles by a metal mesh.

During drilling operation wellbore fluid enters the recess 917 and issubsequently analyzed using sensor unit 916. The results are transmittedfrom the data acquisition unit to the telemetry unit 914, converted intotelemetry signals and transmitted to the surface.

A third application is illustrated in FIG. 10. It shows a Venturi-typeflowmeter 1010, as well known in the industry and described for examplein the U.S. Pat. No. 5,736,650. Mounted on production tubing or casing1012, the flowmeter is installed at a location within the well 1011 witha wired connection 1013 to the surface following known procedures asdisclosed for example in the U.S. Pat. No. 5,829,520.

The flowmeter consists essentially of a constriction or throat 1014 andtwo pressure taps 1018, 1019 located conventionally at the entrance andthe position of maximum constriction, respectively. Usually the Venturiflowmeter is combined with a densitometer 1015 located further up- ordownstream.

The novel electro-chemical analyzing unit 1016 is preferably locateddownstream from the Venturi to take advantage of the mixing effect theVenturi has on the flow. A recess 1017 protected by a metal meshprovides an inlet to the unit.

During production wellbore fluid enters the recess 1017 and issubsequently analyzed using sensor unit 1016. The results aretransmitted from the data acquisition unit to the surface via wires1013.

A further possible application for an embodiment of the presentinvention is in production logging. In order to determine the producingzones of a well, the well is traversed using a logging tool. Forvertical and near vertical wells, the tool is allowed to move undergravity, controlled by a cable from the wellhead. For highly deviatedwells, the tool is pushed/pulled using either coiled tubing from thesurface, or a tractor powered via a cable from the surface.

A typical tool string comprises sensors for taking a series ofmeasurements aimed at determining the flow distribution in the well, interms of phase fractions and position. Measurements include spinners todetermine a local velocity (distribution), and fluid fractionmeasurement probes—for example electrical or optical probes. Thesemeasurements are often used in combination in order to maximize theinformation gained from each pass of the well.

In certain aspects, a pH sensor according to an embodiment of thepresent invention, may be mounted onto this tool, and used to measure pHalong a well. The pH of the aqueous phase may be determined by itscomposition, temperature and pressure and may reveal information on theinflux of fluids into the well as well as the movement of fluids withinthe reservoir. As well as revealing information on fluid influxes andflows within the reservoir, the pH measurement may also be used toassess those parts of the production system that are being exposed tohigh concentrations of acid gases (for which the associated aqueousphase will have a low pH—typically less than about a pH of 4), and arethus prone to corrosion. In certain aspects, this information may beused to determine the strategy for minimizing and/or mitigatingcorrosion, e.g. through the selective placement of corrosion inhibitors.U.S. Pat. No. 6,451,603 to G. M. Oddie describes how sensors might beincorporated within the blades of the spinners within a productionlogging tool and is hereby incorporated by reference in its entirety forall purposes. In certain aspects, a sensor in accordance with anembodiment of the present invention, may be incorporated within theblades of the spinners of a production and may provide for increasingmass transfer to the surface of the sensor.

In another aspect, a sensor in accordance with an embodiment of thepresent invention, may be used in the monitoring of fluids pumped into awell for the purposes of fracturing, matrix treatments such asacidizing, or treatments for wellbore consolidation. pH is an importantparameter that controls the property of some of these fluids, andmonitoring its value may provide a means of assuring the quality of thetreatment, particularly where fluids may be blended in surface modules,prior to being pumped downhole.

In addition to using surface monitoring for pumping fluids into a well,pH might be monitored, in accordance with an embodiment of the presentinvention, on the returns, when a well is brought back on productionfollowing the pumping of a treatment fluid. With the contrast in pHbetween a treatment fluid and the reservoir fluids, the efficacy of thetreatment, and the placement of the treatment fluids, may be assessed.

In yet further aspects, a pH sensor, in accordance with an embodiment ofthe present invention, may be mounted on surface pumping units orblenders, or form part of a separate monitoring module, placed in-lineand/or the like. In still further aspects, a sensor in accordance withan embodiment of the present invention may be deployed downhole on acoiled tubing unit, where the coiled tubing may be used to convey fluidsdownhole, and where the pH sensor may be located at the coiled tubinghead, or as part of a measurement sub conveyed by the coiled tubingunit, and provide information on the state of the fluids downhole.

Another application of an embodiment of the present invention may be inthe monitoring of underground bodies of water for the purposes ofresource management. From monitoring wells drilled into the aquifers,one or more sensors, in accordance with an embodiment of the presentinvention, may be deployed on a cable from the surface—either for shortduration (as part of a logging operation) or longer term (as part of amonitoring application). In certain aspects of the present invention, apH sensor, in accordance with an embodiment of the present invention,may be used in the monitoring of aquifers, where long term unattendedmonitoring of pH is required, e.g. in the monitoring of shallowgroundwater on top of CO2 storage, where the pH in the shallowgroundwater may indicate whether CO2, injected into a deeper aquifer forthe purposes of CO2 sequestration, is escaping to the surface. The pHsensor may be interfaced with a data-logger and the measurements fromthe sensor stored for later retrieval, may be transmitted to surface fordirect analysis and/or the like.

In addition, in certain aspects, the deployment of the pH sensor withinproducing wells on a cable may provide information on produced waterquality. In further aspects, the pH sensor may be deployed in injectionwells, e.g. when water is injected into an aquifer for later retrieval,where pH may be used to monitor the quality of the water being injectedor retrieved.

There may be a significant heterogeneity in the composition, and hencethe pH, of waters produced from a reservoir: reflecting both thevertical and horizontal variations that exist in rock and fluidcomposition. These variations may arise from natural processes duringthe formation of a basin or may come about through the injection offluids to improve oil recovery, e.g., CO2, surface waters or treatmentfluids. Monitoring pH using an embodiment of the present inventionbeyond the wellhead in surface or subsea pipelines may provideinformation on the nature of the flow within a reservoir —providinginformation on events such as water breakthrough or the like—and/or maygive warning when corrosion may become an issue because of abnormallylow pH that may be due to unreacted acid treatments returning to surfaceor because of the natural production of the acid gases H2S or CO2. A pHsensor in accordance with an embodiment of the present invention may bedeployed beyond the wellhead, permanently or temporarily, withinpipelines, or located at the manifolds where pipeline flows are broughttogether or divided.

In wells where reservoir pressures are insufficient, electrosubmersiblepumps (“ESPs”) can be deployed within the well to increase production.These pumps are deployed from surface with a power cable and fluidinjection lines. In certain aspects of the present invention, a pHsensor in accordance with an embodiment of the present invention may bedeployed permanently on the ESP and may provide pH information that maybe used to interpret fluid composition. In such aspects, the sensor mayprovide warning of potential materials failure from acid corrosion orthe like. In addition to this application, alternative means ofdeployment of a sensor in accordance with an embodiment of the presentinvention may be within a permanent monitoring system, may be a part ofa completion of a well and/or the sensors may be deployed through acasing of the wellbore to monitor the fluids outside of the casing, e.g.in assessing zonal isolation or the like.

Various embodiments and applications of the invention have beendescribed. The descriptions are intended to be illustrative of thepresent invention. It will be apparent to those skilled in the art thatmodifications may be made to the invention as described withoutdeparting from the scope of the claims set out below.

1. An electrochemical sensor for measuring an analyte, comprising: afirst working electrode, wherein the first working electrode comprises afirst set of redox species, and wherein the first set of redox speciescomprises one or more redox species that are sensitive to said analyte;a second working electrode, wherein the second working electrodecomprises a second set of redox species, and wherein the second set ofredox species comprises one or more redox species that are insensitiveto said analyte: a counter electrode; a reference electrode; means forapplying a square wave potential sweep between a first and a second pairof electrodes, wherein the first pair of electrodes comprises the firstworking electrode and the reference electrode and the second pair ofelectrodes comprises the second working electrode and the referenceelectrode; and means for detecting relative shifts between a first peakand a second peak in a square wave voltammogram, wherein the first peakis produced by one of oxidation and reduction of the first set of redoxspecies and the second peak is produced by one of oxidation andreduction of the second set of redox species.
 2. The electrochemicalsensor in accordance with claim 1, wherein the means for applying thesquare wave potential sweep between the first and the second pair ofelectrodes comprises at least one of a voltage supply and apotentiostat.
 3. The electrochemical sensor in accordance with claim 1,wherein said analyte comprises protons.
 4. The electrochemical sensor inaccordance with claim 1, wherein the means for detecting relative shiftsbetween the first peak and the second peak in the square wavevoltammogram comprises a signal processor.
 5. The electrochemical sensorin accordance with claim 4, wherein the signal processor processes aconcentration of said certain analyte form relative shifts between thefirst peak and the second peak.
 6. The electrochemical sensor inaccordance with claim 1, wherein the first set of redox speciescomprises one or more redox species sensitive to pH; and the second setof redox species comprises one or more redox species insensitive to pH.7. The electrochemical sensor in accordance with claim 6, wherein theone or more redox species sensitive to pH contain a hydroquinone orquinone moiety.
 8. The electrochemical sensor in accordance with claim6, wherein the one or more species insensitive to pH contain one of aferrocene moiety, a ruthenocene moiety and a hexacyanometallate moiety.9. The electrochemical sensor in accordance with claim 1, wherein thefirst set of redox species is immobilized onto a surface of a conductivesubstrate.
 10. The electrochemical sensor in accordance with claim 1,wherein the first set of redox species is immobilized onto a surface ofa carbon electrode.
 11. The electrochemical sensor in accordance withclaim 9, wherein the immobilization is achieved by direct mixing of thefirst set of redox species with carbon and sealing of the mixture of thefirst set of redox species and the carbon in a binder matrix on theconductive substrate.
 12. The electrochemical sensor in accordance withclaim 9, wherein the immobilization is achieved by direct mixing of thefirst set of redox species with carbon and pressing the mixture of thefirst set of redox species and the carbon into a cavity in theconductive substrate to form a compact layer.
 13. The electrochemicalsensor in accordance with claim 10, wherein the carbon electrodecomprises one of glassy carbon spheres, carbon nanotubes, graphitepowder and boron doped diamond powder.
 14. The electrochemical sensor inaccordance with claim 11, wherein the binder matrix comprises one ofepoxy resin, the redox species sensitive to said analyte and mineraloil.
 15. The electrochemical sensor in accordance with claim 9, whereinthe immobilization comprises direct adsorption.
 16. The electrochemicalsensor in accordance with claim 1, wherein the first working electrodecomprises the first set of redox species chemically attached to aconductive substrate.
 17. The electrochemical sensor in accordance withclaim 16, wherein the chemical attachment is made via a chemicalreduction of a diazo moiety.
 18. The electrochemical sensor inaccordance with claim 16, wherein the first set of redox species iscovalently attached to a surface of the conductive substrate.
 19. Theelectrochemical sensor in accordance with claim 18, wherein the surfacecomprises of a material from the group consisting of one of a basalplane pyrolytic graphite, an edge plane pyrolytic graphite, a glassycarbon, a highly orientated pyrolytic graphite or some combinationthereof.
 20. The electrochemical sensor in accordance with claim 16,wherein the second working electrode comprises a second set of redoxspecies immobilized on a second conductive substrate.
 21. Theelectrochemical sensor in accordance with claim 1, wherein the secondworking electrode comprises the second set of redox species chemicallyattached to a conductive substrate.
 22. The electrochemical sensor inaccordance with claim 1, wherein the counter electrode comprises one ofplatinum, steel or carbon.
 23. The electrochemical sensor in accordancewith claim 1, wherein the reference electrode comprises one of silver(Ag), silver chloride (AgCl) or some combination thereof.
 24. Theelectrochemical sensor in accordance with claim 1, wherein the firstworking electrode and the second working electrode are crossed connectedto a single output of the means for applying the square wave potentialsweep.
 25. The electrochemical sensor in accordance with claim 1,further comprising: a protector configured to protect at least one ofthe first working electrode and the second working electrode, whereinthe protector comprises one of a frit, a fiber mesh, a non-conductingwire mesh, a porous polymer film or some combination thereof.
 26. Adownhole tool for measuring characteristic parameters of wellboreeffluents comprising the electro-chemical sensor in accordance withclaim
 1. 27. The electrochemical sensor in accordance with claim 1,wherein the electrochemical sensor is configured for use in a reservoirsampling tool.
 28. The electrochemical sensor in accordance with claim1, wherein the electrochemical sensor is configured for use in aproduction logging tool.
 29. The electrochemical sensor in accordancewith claim 1, wherein the electrochemical sensor is configured for usein a measurement-while-drilling tool.
 30. The electrochemical sensor inaccordance with claim 1, wherein the electrochemical sensor isconfigured for use in a surface testing module.
 31. The electrochemicalsensor in accordance with claim 1, wherein the electrochemical sensor isconfigured for use in a surface pumping module, the surface pumpingmodule delivering well treatment fluids.
 32. The electrochemical sensorin accordance with claim 1, wherein the electrochemical sensor isconfigured for integration into part of a sensor string installedpermanently or temporarily within a monitoring, production, or injectionwell in aquifers.
 33. The electrochemical sensor in accordance withclaim 1, wherein the electrochemical sensor is configured to bepermanently installed in a pipeline for transporting hydrocarbons. 34.The electrochemical sensor in accordance with claim 1, wherein theelectrochemical sensor is configured to be permanently installed on adownhole pump.
 35. The electrochemical sensor in accordance with claim1, wherein the electrochemical sensor is configured for use on one of adownhole treatment monitoring tool for stimulation or formation damageremoval, an end of a tubing section, a drillpipe and a coiled tubingunit.
 36. The electrochemical sensor in accordance with claim 1, whereinthe electrochemical sensor forms part of a permanent monitoring systemwithin a well, the sensor being disposed in the well behind casing. 37.The electrochemical sensor in accordance with claim 1, wherein theelectrochemical sensor is configured to monitor CO2 sequestration.
 38. Amethod for measuring a certain analyte, comprising: contacting a firstworking electrode, a second working electrode and a counter electrodewith a fluid comprising said analyte, wherein the first workingelectrode comprises a first set of redox species sensitive to saidanalyte and the second working electrode comprises a second set of redoxspecies insensitive to said analyte; applying a first square wavepotential sweep between the first working electrode and a referenceelectrode; applying a second square wave potential sweep between thesecond working electrode and a reference electrode; determining a firstpotential on the square wave potential sweep, wherein the firstpotential corresponds to a first peak current flow between the firstworking electrode and the counter electrode, and wherein the first peakcurrent flow is produced by oxidation or reduction of the first redoxspecies; determining a second potential on the square wave potentialsweep, wherein the second potential corresponds to a second peak currentflow between the second working electrode and the counter electrode,wherein the second peak current flow is produced by oxidation orreduction of the second redox species; and using the relative separationbetween the first and second potentials to measure the certain analyte.39. The method of claim 38, further comprising: cross-connecting thefirst and second working electrode, wherein the first and the secondsquare potential sweeps comprise a single square wave potential sweep.